Stereophonic loudspeaker arrays



March 5, 1963 B. B. BAUER STEREOPHONIC 'LOUDSPEAKER ARRAYS Filed Sept.26, 1958 2 Sheets-Sheet 1 FIG.

INVENTOR. BENJAMIN B. BAUER his ATTORNEYS March 5, 1963 B. B. BAUER3,080,012

STEREOPHONIC LOUDSPEAKER ARRAYS Filed Sept: 26, 1958 2 Sheets-Sheet 2INVENTOR. BENJAMIN B. BAUER A T TOFIWE YS 3,tl8tl,012 STEREGPHGNIQLQUDSPEAKER ARRAYd Beniamin B. Bauer, Stamford, 60nd, assignor toColumhia Broadcasting System, inc, New York, N.Y., a corporation of NewYork Filed Sept. 26, 1953, Ser. No. 763,499 8 Clmms. (@l. 1813i) Thisinvention is directed to loudspeaker arrays for stereophonic playbacksystems and, more particularly, to a new and improved loudspeakerarrangement adapted to produce a correct stereophonic impression over abroad listening area.

in conventional two-channel stereophonic systems, two loudspeakers areemployed for the reproduction of the individual right and left channelsand these are generally placed against one wall of a room at a distancefrom each other which is equal to about 70% of the length of the wall.Any listeners on the axis of symmetry between the two loudspeakers willhear both channels equally Well and perceive a correct stereophonicimpression. Observers oil the axis of symmetry, however, will hearpredominantly the sound from the loudspeaker which is closest to them,and this effect provides a distorted perspective of sound.

It the microphones used in recording a two-channel stereophonicperformance are highly directional or are placed close to individualperformers of a group, there will be considerable separation between thesounds of the two channels and upon reproduction, the sounds will emergepredominantly from one or the other loudspeaker. Since these sounds arebasically unlike, they will be perceived as two different sounds, therebeing no apparent sound coming from the space in between the twoloudspeakears creating what is known as a hole. In order to fill outthis hole, the two microphones are usually directed to receive equallythe center sounds. In addition, a third microphone arranged to receivesound from performers at the center of the group and its output is fedequally into the two channels of the stereophonic system. Even with thisarrangement, however, observers on the axis do not perceive the centersounds correctly, but are subject to a shifting center efiect wherebythe origin of a sound from a centrally located source appears to besubstantially closer to the loudspeaker nearest the observer because ofthe greater intensity of sound received from that speaker.

Accordingly, it is an object of this invention to provide an improvedstereophonic system in which the loudness of the left and right channelsare about equal for all observers within the normal listening area.

Another object of the invention is to provide a loudspeaker systemcapable of conveying an illusion that a centered sound originates fromthe space between the two loudspeakers, for all the observers in theroom.

Still another object of the invention is to provide a stereophonicloudspeaker, system in which substantially equal stereophonic perceptionis obtained by all of the observers in the listening area.

A further object of the invention is to provide a loudspeaker enclosurecapable of producing a variable radiation pattern for use in accordancewith the invention.

These and other objects of the invention are attained by directing thetwo loudspeakers of a stereophonic system so that their axes intersectat a point in front of the listening area. According to the invention,the pattern of sound radiation from these loudspeakers, which decreasesin intensity as the angle from the axis of the speaker increases, isutilized to produce substantially equal loudness from both speakers atany point in the listening area when identical signals are applied tothe hired h t-sates i dlifiiliii il Patented Mar. 5, 1963 two speakers.inasmuch as only the middle frequencies and the high frequencies ofsound are primarily eflfective to produce a stereophonic impression,these frequencies may be selected for reproduction over the twoloudspeakers and the remainder of the sound spectrum can be applied to acentrally located loudspeaker in another embodiment of the invention.

In order to improve the stereophonic etiect provided in the abovemanner, angled loudspeakers having a radiation pattern controlled byselection of the acoustic resistance of the enclosure may be utilizedfor each of the two stereophonic channels. In this embodiment, theloudspeaker enclosure includes a wall of perforate material covered withfabric behind a speaker, the acoustic resistance of the material and thevolume of the enclosure being selected to shift the phase of the soundwithin the enclosure to obtain any desired pattern of radiation. Inaddition, a variable shutter allowing for adjustment of the exposed areaof the fabric may be used to vary the acoustic resistance and the entireenclosure may be covered with a sound permeable but substantially opaquematerial to prevent listeners from observing the angular orientation ofthe loudspeaker.

Further objects and advantages of the invention will be apparent from areading of the following description in conjunction with theaccompanying drawings in which:

FIG. 1 is a schematic plan View of a typical loudspeaker system arrangedaccording to the invention;

FIG. 2 is a plan view in section showing a representative enclosure fora loudspeaker utilized for stereophonic reproduction;

FIG. 3 is a plan view of another form of loudspeaker arranged accordingto the invention;

FIG. 4- is a schematic view illustrating the radiation pattern generatedby one form of loudspeaker arranged according to the invention;

FIG. 5 is a plan view illustrating a stereophonic system employingloudspeakers of the type shown in FIG. 4; and

FIG. 6 is a sectional view illustrating the structure of a form ofloudspeaker capable of generating a variable sound radiation pattern.

One form of loudspeaker array according to the in-' vention is shown inFIG. 1 wherein a room It; includes a front portion 11, which may be astage, for example, whereon two enclosed loudspeakers i2 and 13receiving the left and right channels, respectively, of a stereophonicsignal are positioned. The remainder of the room behind the front line14- of the stage comprises a listening area 15 and may include theseating arrangement usually found in an auditorium, for example. On theother hand, the room it may be small, such as the living room of a home,in which case the line 14 is merely an imaginary line extending acrossthe width of the room between the speakers 12 and 13 and the nearestlistener.

Instead of being placed flat against the end wall 16 of the room in theusual manner, the loudspeaker enclosures .are shifted at a considerableangle so that the axes of the two loudspeakers intersect at a point 17located generally forward of the line 14, forming an obtuse angle 0 atthe point of intersection. Experiment has shown that the optimum valueof 0 varies with the size of the loudspeaker and the enclosures. Forloudspeakers having a cone diameter of 15 inches and housed inenclosures 2 feet Wide, the optimum angle 0 is approximately l20. For 8inch diameter loudspeakers housed in enclosures about 1=2 inches wide,the angle 0 should be approximately for the best stereophonicreproduction. Where 5 inch loudspeakers the housed in enclosures 8inches wide, the optimum angle 0 is almost 180, but as the angleapproaches 1180" the stereophonic efiect becomes less evident.

It is apparent, therefore, that the improvement observed in theseexperiments stems from certain properties of the loudspeakers dependenton their size. More particularly, a loudspeaker mounted in an enclosureexhibits directional properties which are different at differentfrequencies. At very low frequency, when the wave length of sound islarge compared to the dimension of the loudspeaker cone, the waveemanating from the loudspeaker has uniform intensity in all directions,as shown by the dash circles 18 and 13' in FIG. -l. At the higherfrequencies for which the wave length of sound is comparable to orsmaller than the diameter of the diaphragm, the loudspeaker radiatespreferentially along the axis as shown by the outlines l9 and 19', thelength of .a radius vector from the center of the loudspeaker cone tothe outline portraying the sound pressure radiated in the direction ofthe radius vector. It is well known that stereophonic perception is moreevident at higher frequencies than at lower frequencies and, therefore,it is this sharpening of the radiation pattern at high frequency that isresponsible for the improved stereophonic effect, if the loudspeakersare properly oriented according to the invention.

In operation, when an observer is at a position 20 on the longitudinalcenter line 21 of the room the sound pressure radiated in his directionfrom the two speakers 12 and 13, which are equidistant from him, isshown by the vectors 22 and 22', respectively. Because of symmetry,these vectors are of equal length if equal signals are applied to thetwo speakers as by a centrally located source of sound, and the observerwill hear both loudspeakers with the same loudness. If the observermoves to a position 23 near one side of the room, the intensity from theloudspeaker 12 is represented by the vector 24 which is longer than theformer vector 22 and therefore, corresponds to a greater intensity ofsound. In this position, however, the observer is farther from theloudspeaker 12 than before so that the loudness of the sound he hearsfrom this loudspeaker is substantially the same as before. At the sametime, the intensity of sound directed toward the position 23 by theloudspeaker 13 is less than that directed toward the position 20, asrepresented by the shorter vector 24'. However, at the position 23 theobserver is closer to the loudspeaker 13 than when he was at the centralposition 20 so that the sound he hears from this speaker hassubstantially the same loudness as at the position 20. Therefore, thesounds from the two loudspeakers 12 and 13 arriving at the position 23will be equally loud, thus giving the observer the required degree ofstereophonic perception. By proper inclination of the loudspeakersaccording to the invention, this satisfactory perception is obtainedpractically everywhere in the room. At the same time, the centersou'nds, which are transmitted over the two channels with equalintensity, appear to come from an angle intermediate between the twoloudspeakers, i.e. on the'line 25, and an apparent central sound sourceis perceived by the observer regardless of his position in the'listeningarea 15.

Inasmuch as the inclination of the loudspeaker axes at an angle 6 isimportant to the production of the desired effect as described above, Iprefer to provide a special enclosure for the loudspeakers whichautomatically positions them at the desired angle. Such an enclosure isshown in FIG. 2 wherein the numeral 26! closures are placed flat againstthe end wall 16', as shown in FIG. 3, the angle of intersection of thetwo axes of the speakers will be equal to i=2 (1:. For a loudspeakerwith an 8 inch cone the desired angle is found to be approximately 70,giving a total angle of intersection 0 of Both the enclosures 26 and 26'are preferably provided with a foraminous grille or cover 32 which doesnot allow the observer to see the direction in which the loudspeaker ispointing. This is an important psychological effect which greatlycontributes to the effectiveness of the loudspeaker system.

Since it has been shown that stereophonic effect is mainly caused by themiddle range of the high frequencies of sound, the stereophonicloudspeaker arrangement may be very effectively utilized in combinationwith the socalled mixed lows system. According to this system, theoutputs of all stereophonic channels at the frequencies below a certainrange, say 10 0300 c.p.s., are mixed together and reproduced over oneloudspeaker, while the frequencies above this range are kept separateand are reproduced over two other separate loudspeakers. In this manner,the high frequency loudspeakers may be quite small, say 10 inches, andyet capable of providing the improved stereophonic effect describedherein.

An example of this arrangement is shown in FIG. 3 in which theloudspeaker 33 is designed to reproduce the mixed frequencies of bothchannels up to 300 c.p.s., and the loudspeakers 31 and 31 reproduce thehigh frequency stereophonic information of the left and right channels,respectively, which is above 300 c.p.s. In this case, the enclosures26and 26 may each be approximately a 10 inch cube. Also, the loudspeaker33 may be placed anywhere in the room because the origin of the lowfrequency does not play an important part in producing the stereophonicperception. The enclosures 26 and 26' are placed against the wall sothat the axes 34 and 3 4 of the speakers 31 and 31' intersect in thepredetermined angle 0 at a point 35 in front of the line 14 to operatein the manner described above. i v

Inasmuch as the radiation pattern of a loudspeaker contributes to theimproved stereophonic impression, the use of directional loudspeakers isespecially valuable in carrying out the purposes of the invention. Adirectional loudspeaker may be obtained by taking a conventional movingcoil loudspeaker and mounting it in a bafile of suitable design. If anopen cone loudspeaker 36 is mounted in a small open baffle 37 as shownin FIG. 4, for example, then the radiation pattern at all frequencies upto and including those at which the wave length equals twice thedimensions of the baflle will follow the cosine law (p==cos 0) shown inthe drawing by the two dash circles 38 and 39. I have found this to be avery desirable radiation pattern to carry out the purposes of myinvention. Thus, FIG. 5 shows a schematic arrangement which employs twoloudspeakers 40 and 41 similar to those in FIG. 4, to reproducefrequencies above 250 c.p.s., for example, the open baffles beingrepresented by the numerals 42 and 43. The loudspeaker 40 receivesinformation from the left stereophonic channel above 250 c.p.s., and theloudspeaker 41 receives the energy from the'right stereophonic channelabove the frequency 250 c.p.s., and the two front radiation patterns ofthese loudspeakers are shown in the drawing by the dash circles 44 and45. Both of these loudspeakers and their bafiies 42 and 43 arepreferably enclosed in foraminous enclosures 50 and 51 to conceal thefact that the loudspeakers are inwardly oriented for the purpose pointedout above. Referring again to FIG. 4, it may be observed that theradiation pattern 38 tends to narrow down at high frequency withspeakers of this type. This effect, however, may be diminished byplacing a suitable grille or an acoustical lens 52 comprised ofappropriate baffles 53 of varying length in front of the loudspeaker, orby using a dual cone loudspeaker, as is well known in the art.

In operation, it will be apparent that an observer at the position 23receives practically the full amplitude of sound radiated by theloudspeaker 40 as portrayed by the vector 46 while at the same time hereceives a much lower amplitude from the loudspeaker 41, as shown by thevector 47. However, since he is so much nearer to the loudspeaker 41,the sounds from both the loudspeakers appear to him to be about equal inloudness. By the same token, the center sounds appear to originate froma central point between the two speakers on the line 48 as describedabove with reference to FIG. 1. It may be shown by analysis that thesimilarity of intensity from the two channels employing this system iswithin less than 2.5 db over practically the entire listening area 15 ofthe room. As in the system shown in FIG. 3, the frequencies from bothchannels in the range below 250 c.p.s. are mixed together and reproducedfrom a single speaker 33.

Inasmuch as the loudspeakers 40 and 41 are mounted in open bafiles 42and 43 and are not enclosed at the rear, the sound radiated from theback of the loudspeakers is thoroughly dispersed and mixed in the roomand it gives an added reverberant effect which creates the illusion ofbeing in a concert hall.

Another useful type of enclosure for use with loudspeakers in accordancewith my invention is one which will produce a cardioid radiation pattern(0.5 +0.5 cos 9) or any other pattern in the limacon family (1K+K cos0), where O K L An example of such an enclosure is shown in crosssection in FIG. 6 wherein the enclosure comprises a box 54 of agenerally parallelepipedal shape having one wall 55 carrying aloudspeaker 56 and inclined at an angle in the manner described abovewith respect to FIG. 2. Opposite the wall 55, a rear wall 57 is made ofperforated material which may be covered with a layer of fabric 60, andbehind this wall a variableshutter 58, slidable in a groove 59, forexample, allows for adjustment of the exposed area of the fabric and,therefore, its acoustic resistance. The acoustic resistance of fabric 60and the volume of the box 54 are selected to shift the phase of soundwaves inside the box in such a manner that any desired pattern of soundradiated by the loudspeaker is obtained, such as a cardioid pattern asshown in the drawing by the dash line 61. At high frequency this patternwill sharpen as described before to approximate a circle 62.

In operation, two of the enclosures 54 which are mirror images of eachother are arranged against an end wall 16 in the manner shown in FIG. 3.After installation in this manner the shutters 58 are adjusted toachieve the optimum ratio of direct-to-random sound to suit thereverberant conditions of the room. Also, an improved three-channelstereophonic system may be obtained by using two loudspeakers inenclosures 54 as hereinbefore described as the left and right channelloudspeakers respectively along with a conventional loudspeaker for thecenter channel of the system.

Although the invention has been described herein with reference tospecific embodiments, many modifications and variations therein willreadily occur to those skilled in the art. Accordingly, all suchvariations and modifications are included within the intended scope ofthe invention as defined by the following claims.

I claim:

1. A stereophonic enclosure and loudspeaker arrangement comprising afirst area having at least two substantially equal loudness loudspeakersarranged along one side of said first area, said loudspeakers beingarranged with their axes at at least 100 but less than 180 with respectto each other, said axes intersecting at a point within said first area,said enclosure having a second area for listeners, the sound from saidloudspeakers in said second area being substantially the same for anysounds of equal intensity coming from said loudspeakers.

2. A stereophonic enclosure and loudspeaker arrange- 6 ment according toclaim 1 wherein each of the loudspeakers has a frequency range extendingabove 300 cycles per second.

3. A stereophonic enclosure and loudspeaker arrangement according toclaim 2 including a further loudspeaker having a frequency rangeextending below 300 cycles per second.

4. A stereophonic enclosure and loudspeaker arrangement according toclaim 1 wherein each of the loudspeakers includes means for radiatingacoustic energy directionally so that the radiation intensitysubstantially follows a limacon relation as the angle from the axis ofthe loudspeaker varies.

5. A stereophonic enclosure and loudspeaker arrangement according toclaim 4 including housing means for each loudspeaker having an openingbehind the loudspeaker so that acoustic energy radiated forwardly andrearwardly from the loudspeaker is combined in the proper magnitude andphase to produce a radiation intensity which substantially follows alimacon relation as the angle from the axis of. the loudspeaker varies.

6. A stereophonic enclosure and loudspeaker arrangement according toclaim 5 including acoustic resistance means covering the opening in thehousing means behind the loudspeaker to control the intensity of theacoustic energy radiated rearwardly therefrom.

7. A stereophonic enclosure and loudspeaker arrangement according toclaim 5 including movable shutter means at the rear of the housing meansand adjacent to the opening therein to control the size of the openingand vary the intensity of the rearwardly radiated acoustic energy.

8. A stereophonic enclosure and loudspeaker arrangement according toclaim 4 wherein the housing means includes a bafile having an opening inwhich the loudspeaker is mounted, said bafiie being only slightly largerthan the diameter of the loudspeaker.

References Cited in the file of this patent UNITED STATES PATENTS1,645,231 Dietrich et al Oct. 11, 1927 1,855,147 Jones Apr. 19, 19321,902,609 Beers et a1 Mar. 21, 1933 2,065,751 Scheldorf Dec. 29, 19362,137,032 Snow NOV. 15, 1938 2,210,477 Benecke et al. Aug. 6, 19402,520,798 DeBoer Aug. 29, 1950 2,580,439 Kock Ian. 1, 1952 2,623,606Corke Dec. 30, 1952 2,710,662 Camras June 14, 1955 2,766,839 Baruch eta1 Oct. 16, 1956 2,858,899 Lopez-Henriquez Nov. 4, 1958 2,904,124 LyonsSept. 15, 1959 2,924,660 Abrams Feb. 9, 1960 FOREIGN PATENTS 102,888Australia Dec. 23, 1937 197,911 Switzerland May 31, 1938 229,642Switzerland Feb. 16, 1944 641,668 Great Britain Aug. 16, 1950 955,960Germany Jan. 10, 1957 957,044 Germany Jan. 31, 1957 1,142,072 FranceMar. 25, 1957 781,637 Great Britain Aug. 21, 1957 OTHER REFERENCESPublication A Stereophonic Magnetic Recorder by Marvin Camras, pages 445and 446, April 1949.

Textbook, Acoustical Engineering, by H. F. Olson, copyright 1957, by D.Van Nostrand Co., Inc.

1. A STEREOPHONIC ENCLOSURE AND LOUDSPEAKER ARRANGEMENT COMPRISING AFIRST AREA HAVING AT LEAST TWO SUBSTANTIALLY EQUAL LOUDNESS LOUDSPEAKERSARRANGED ALONG ONE SIDE OF SAID FIRST AREA, SAID LOUDSPEAKERS BEINGARRANGED WITH THEIR AXES AT AT LEAST 100* BUT LESS THAN 180* WITHRESPECT TO EACH OTHER, SAID AXES INTERSECTING AT A POINT WITHIN SAIDFIRST AREA, SAID ENCLOSURE HAVING A SECOND AREA FOR LISTENERS, THE SOUNDFROM SAID LOUDSPEAKERS IN SAID SECOND AREA BEING SUBSTANTIALLY THE SAMEFOR ANY SOUNDS OF EQUAL INTENSITY COMING FROM SAID LOUDSPEAKERS.